1. Field of the Invention
This invention relates to a facility for generating fibers, in particular mineral fibers, from a molten mass.
2. Description of Related Art
During blast drawing, primary fibers emerging, as a rule, from a series of openings in a molten mass separating device are fed to the drawing gap of a blast pipe. They are then accelerated in a flow of gas that is fed to the drawing gap at the same time and, thus, they are pulled. The speed profile of the flow illustrates marked stress along the borders, and the high-speed currents along the borders are generated, as a rule, by a blast flow introduced along the borders, which also introduces into the facility the energy required for the extraction process. Due to the peak speeds reached at the borders and the ensuing vacuums, the primary fibers are subjected to lateral excursion, causing them to swing quickly between the two high-speed zones along the borders. In the process, parts of the fibers time and again come under the influence of the high-speed currents at the border and are swept along, while fiber parts lying in between bridge-over the gap between the high-speed zones along the borders. In this way, effects resembling the cracking of a whip result, in which the fiber parts--under the influence of various flow speeds--are subjected to tensile force and, thereby, are pulled into thin fibers.
Behind the opening of the blast pipe the gas-fiber-dispersion has to be delayed, and the static pressure of the gas flow has to be raised to about the ambient pressure so that the fibers, which become compact when they are cooled off, can, finally, be deposited to form a fibrous web. A subsonic diffuser can be used as a delaying device. In this connection, it must be noted that the flow in the subsonic diffuser exhibits no large swirls or other cross currents, which could move the fibers towards each other and towards the edge of the flow. This is necessary because when fibers that are not yet compact touch each other or the wall, non-pulled material parts (beads) are formed, which, although they do contribute to the overall density of the finished product, cannot assume the function of fibers. If parts of fibers that are not yet sufficiently compact do touch each other, then the fiber, which is under tensile stress, tends to tear at the point of contact, causing the prevailing tensile stress to be eliminated. Moreover, the fiber end tends to recoil into a spherical form because the surface tension of molten masses that are to be processed after blast drawing is many times greater than that of water, and if there are no external forces, it always exhibits the tendency to generate a spherical form with minimal surface per volume. If a fiber part that is not yet completely compact touches the wall, then it is abruptly stopped, also tears, and takes on a spherical form.
To avoid such cross currents, it must be safely ensured that the flow cannot detach itself from the flow border of the subsonic diffuser. Since the flow speed and, thus, also Reynolds, number are very high, safe avoidance of detachment when the flow is delayed in the subsonic diffuser presupposes a very slight opening angle of, at the most, a few degrees.
A good efficiency factor and, thus, slight energy consumption is achieved due to the low-loss pressure conversion in such a narrow subsonic diffuser. Furthermore, the formation of fine, strongly warped fibers is favoured by the fact that the speed profile from the blast pipe is maintained, in terms of quality, over a long stretch in the subsonic diffuser and that the lateral low-pressure zones suck in neighboring fiber parts and subject them to a relative acceleration. For this reason, they provide an additional pulling effect until the parts are compact.
However, the ensuing cross movements of the fiber parts in the direction of the flow border increase the probability of wall contact in view of the slight opening angle, and, thus, there is also increased bead formation caused by such wall contact. Furthermore, the fibers are led in close vicinity to each other over a long stretch so that even slight relative movements can lead to mutual contact and, therefore, also to bead formation. Wall contact and mutual contact of compact fibers are also probable; this leads to a reduction in the median fiber length because the fibers break. For this reason, it is difficult to produce fine fibers with a great median fiber length and a slight share of beads by means of blast drawing.